The present invention relates generally to radiation therapy, and more particularly, to a method and system for extending the field area of an intensity map used in radiation therapy with a modulating multi-leaf collimator.
Radiation emitting devices are generally known and used, for instance, as radiation therapy devices for the treatment of patients. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located within the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam may be an electron beam or photon (x-ray) beam, for example. During treatment, the radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation.
In order to control the radiation emitted toward the patient, a beam shielding device, such as a plate arrangement or collimator, is typically provided in the trajectory of the radiation beam between the radiation source and the patient. An example of a plate arrangement is a set of four plates which can be used to define an opening for the radiation beam. The collimator is a beam shielding device which may include multiple leaves (e.g., relatively thin plates or rods) typically arranged as opposing leaf pairs. The plates are formed of a relatively dense and radiation impervious material and are generally independently positionable to delimit the radiation beam.
The beam shielding device defines a field on the zone of the patient for which a prescribed amount of radiation is to be delivered. The usual treatment field shape results in a three-dimensional treatment volume which includes segments of normal tissue, thereby limiting the dose that can be given to the tumor. The dose delivered to the tumor can be increased if the amount of normal tissue being irradiated is decreased and the dose delivered to the normal tissue is decreased. Avoidance of delivery of radiation to the healthy organs surrounding and overlying the tumor limits the dosage that can be delivered to the tumor.
The delivery of radiation by a radiation therapy device is typically prescribed by an oncologist. The prescription is a definition of a particular volume and level of radiation permitted to be delivered to that volume. Actual operation of the radiation equipment, however, is normally done by a therapist. The radiation emitting device is programmed to deliver the specific treatment prescribed by the oncologist. When programming the device for treatment, the therapist has to take into account the actual radiation output and has to adjust the dose delivery based on the plate arrangement opening to achieve the prescribed radiation treatment at the desired depth in the target.
The radiation therapist""s challenge is to determine the best number of fields and intensity levels to optimize dose volume histograms, which define a cumulative level of radiation that is to be delivered to a specified volume. Typical optimization engines optimize the dose volume histograms by considering the oncologist""s prescription, or three-dimensional specification of the dosage to be delivered. In such optimization engines, the three-dimensional volume is broken into cells, each cell defining a particular level of radiation to be administered. The outputs of the optimization engines are intensity maps, which are determined by varying the intensity at each cell in the map. The intensity maps specify a number of fields defining optimized intensity levels at each cell. The fields may be statically or dynamically modulated, such that a different accumulated dosage is received at different points in the field. Once radiation has been delivered according to the intensity map, the accumulated dosage at each cell, or dose volume histogram, should correspond to the prescription as closely as possible.
Conventional treatment planning systems are designed to only allow for radiation delivery with intensity maps having dimensions generally equal to the number of leaves*leaf width in the direction perpendicular to leaf motion and equal to the (leaf over travel*2)+(pencil beam width*2) in the leaf travel direction. These field sizes are the results of mechanical limitations at a collimator setting for arbitrary intensity distributions. Thus, conventional systems are limited to a fixed field size, collimator setting, and arbitrary intensity distributions.
There is, therefore, a need for a system and method that allows for field sizes that are beyond the field sizes imposed by mechanical constraints of a multi-leaf collimator.
A method for defining an extended field area of an intensity map for use in delivering radiation from a radiation source to an object with a multi-leaf collimator is disclosed. The multi-leaf collimator includes a plurality of leaves operable to travel in a first direction and is rotatable such that the leaves are operable to travel in a second direction extending generally orthogonal to the first direction. The method generally comprises defining a central square area having dimensions approximately equal to two times an over travel margin of the multi-leaf collimator and defining four edge margins each extending from a side of the central square and having dimensions approximately equal to two times an over travel margin of the multi-leaf collimator along an edge adjacent to the central square, and half the number of leaves within the multi-leaf collimator times leaf width minus half the central square dimension. The central square and four edge margins define a field area for an intensity map deliverable with the multi-leaf collimator positioned such that leaves travel in the first and second directions.
A system for defining an intensity map for use in delivering radiation from a radiation source to an object using a multi-leaf collimator generally comprises a processor operable to define a central square area having dimensions approximately equal to two times an over travel margin of the multi-leaf collimator and define four edge margins each extending from a side of the central square and having dimensions approximately equal to two times an over travel margin of the multi-leaf collimator along an edge adjacent to the central square, and half the number of leaves within the multi-leaf collimator times leaf width minus half the central square dimension. The central square and four edge margins define a field area for an intensity map deliverable with the multi-leaf collimator positioned such that leaves travel in the first and second directions. The system further includes memory configured to at least temporarily store the intensity map.
A method for delivering radiation from a radiation source to an extended field area with a multi-leaf collimator generally comprises creating an intensity map having boundaries defined by two rectangles each having dimensions approximately equal to two times an over travel margin of the multi-leaf collimator and the number of leaves of the multi-leaf collimator times leaf width. The two rectangles are arranged such that the center of the rectangles have the same central axis and are positioned orthogonal to one another.
The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.